Screen printing is a well established and substantial industry. Essentially, printing screens are imparted with various designs, art work or printed indicia by rather permanent emulsions on the screen for localized application of dye pastes or inks used in reproducing the image from the screen. The emulsions surrounding the image areas of the printed screens are resistant to inks so that they resist removal during the printing process when ink is applied through the screen for reproduction of the images therefrom. The same qualities which make emulsions resistant to ink and cleaning solvents can make them more difficult to be removed from a printed screen. Printing screens are usually made from silk, synthetic fabric or metal materials and, in the practice of screen printing, it is common to reuse them. This involves a cleaning process whereby ink residue from one printing operation will be removed and cleaned from the screens which permits their storage and later reuse. In the removal of ink, it is sometimes important not to affect the emulsion area which has been imparted to the screen. Modern day screen printing has evolved rather complex ink or dye formulations which are sometimes very difficult to remove. Agents which may be suitable to clean inks from the screen may also affect the underlying emulsion. Therefore, screen printing involves a balanced variety of chemical processes in which screens are prepared from artwork with semi- to fully-permanent emulsions for reproducing ink images in a matter such that the emulsion areas resist ink attack. In such operations, the screen is repeatedly cleaned for filing and/or subsequent reuse. It is also important that the screen be capable of reclamation when ink image and/or emulsion areas are removed with different types of screen reclaiming solutions or agents.
Commercial screen printing shops usually clean or reclaim many screens daily and, for this purpose, employ screen cleaning machines or reclaiming systems. Such cleaning machines or reclaiming systems usually employ recirculating solvents which allow the synthetic or metal screens to be introduced and either cleaned or reclaimed, depending upon production requirements. In addition, other commercial operations involve hand-cleaning or reclaiming with various solvents or corrosive agents. During the course of cleaning or reclaiming, the screen printer is often in intimate contact with the chemicals or solvents. There are many solvents or agents used in screen cleaning and reclamation. The three most commonly used agents may be classified as aliphatic hydrocarbons, aromatic hydrocarbons and oxygenated solvents, and less frequently, chlorinated solvents. Aliphatic hydrocarbons are commonly referred to as "mineral spirits" but more accurately these aliphatic solvents are composed of mixtures of straight-chain and/or branched-chain saturated hydrocarbons. The higher the molecular weight or the number of carbon atoms, the higher the boiling point of the solvent. The higher the boiling point, the slower the solvent will evaporate, hence, usually the higher boiling aliphatic hydrocarbons are desirable for use in screen cleaning. The aromatic hydrocarbon solvents include cyclic hydrocarbons containing the benzene ring. These aromatic hydrocarbons are usually more flammable but much stronger in solvation power than the aliphatic type solvents and, similarly, the higher the molecular weight for the aromatic hydrocarbon, the higher the boiling range. In contrast to the rather non-polar hydrocarbons, oxygenated solvents are more polar compounds. Typically, oxygenated solvents are those having hydroxyl or carbonyl groups, and many of them have considerable solubility in water. Other solvents include chlorinated solvents which are fully- or semi-chlorinated hydrocarbons and the rarely used fluorinated hydrocarbons of the Freon types.
Today's screen printer thus routinely deals with a multiplicity of solvents which are used for screen cleaning and reclamation. In the past, when there was perhaps little understanding of the health and safety hazards which printers or workers were exposed to, such solvents were used with impunity. More recently, in view of Federal and State legislation, solvents must meet material safety standards. Prolonged or repeated solvent contact with the skin is generally avoided and, in most instances, because of flammability, solvents must be kept away from extreme heat or open flame, and frequently fire departments request their storage outside. Occupational Safety and Health Administrations at both Federal and State levels have also placed various restrictions upon the use of solvents and, many may no longer be used. Furthermore, in know processes, hot reclamation systems have been required in order to clean to reclaim printing screens, but such systems create pollution and hazards which are no longer tolerable. In the search for suitable screen cleaning and reclamation solvents or agents, it would be desirable to be able to eliminate the DOT (Department of Transportation) red label which warns of hazardous, flammable solvents. It would also be desirable to offer cleaning and reclamation compositions which are either completely or essentially biodegradable. Another highly desirable objective would be to make available to the industry cleaning and reclamation products having a high threshold limit values (TLV) which means that the amount of airborne matter provided by such products offers greater safety in breathing.
The above background provides a practical overview of the screen printing industry from the standpoint of the cleaning and reclamation processes to a person of ordinary skill in this art. In addition, in the preparation of this application, patents have been located which may be considered to relate to the subject matter of this invention. The following is a list of prior patents which amy be helpful in understanding this invention without leaving the impression that it is exhaustive or that there may not be more relevant patent art or literature: U.S. Pat. Nos. 2,780,168; 3,459,594; 3,511,657; 3,615,827; 3,642,537; 3,673,099; 3,678,479; 3,706,691; 3,737,386; 3,764,384; 3,789,007; 3,796,602; 3,928,065; 3,953,352; 4,024,085; 4,055,515 and 4,070,203. It must be mentioned that these patents have been listed with the knowledge of this invention and even have been obtained from non-analogous arts. Therefore, it is not to be in any way inferred that their listing here represents the state of the printing screen cleaning or reclamation art.
It is submitted that there is a need for printing screen cleaning and reclamation compositions which are effective in a wide variety of applications. Moreover, it is highly desirable that such compositions, while effective, nevertheless, meet environmental health and safety standards.